A memory module for a computer or computer-based device generally comprises a circuit board having dynamic random access memory (DRAM) chips and a connector that enables communication with a motherboard. To operate successfully, a memory module typically meets standard timing and interface requirements for the type of memory module intended for use in the particular computer. These requirements may be proprietary, and/or defined in design specification documents that are published by either the original initiator of the standard (e.g., INTEL or IBM) or a standards issuing body such as JEDEC (Joint Electron Device Engineering Coucil).
DRAMs used in memory modules are often identified as ×4 or ×8 DRAMs. The distinction between ×4 and ×8 is determined by different number of data outputs per DRAM, with the total amount of memory available per memory module being the same. For example, error correction code (ECC) memory modules often feature 72 data bits (64 data bits plus 8-ECC bits). Therefore, a single-rank memory module with ×4 devices uses 72/4 or 18 total DRAM chips. Memory modules featuring ×8 devices use 72/8 or nine total chips. The 72-bit unit of devices (18 or 9) is referred to as a rank. In other words, rank is a term used to refer to the set of DRAM devices that are accessed during a single memory transfer. For example, the number of devices accessed is equal to the size of the data bus divided by the device width of the DRAM. A single chip select is common for all the devices in a single rank. Memory modules may also comprise two ranks, and are sometimes referred to as high density memory modules.
To respond to consumer demand for higher capacity memory modules, manufacturers of memory modules have attempted to place a higher density of memory integrated circuits on printed circuit boards. One mechanism for achieving high memory density is through the use of micro-ball grid array (micro-BGA) designs. Micro-BGA integrated circuits use a connection technique that places the connections for the integrated circuit between the body of the integrated circuit and the surface of the printed circuit board. Stacking is another technique, whereby a second layer of integrated circuits is provided on top of the integrated circuits disposed upon the surface of the printed circuit board.
The demand for high speed, high capacity memory modules for use in the computer industry has grown rapidly, fostering the need for continued improvements in these and other memory module designs and techniques.